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Post subject: Toasted SF Dual Showman
Posted: Fri Mar 15, 2013 5:31 pm
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Kinda an add-on to the "Problems with the Twin Reverb" thread. Some photos of Showman I'm currently trying to fix.

Note the wrong resistor in the bias supply (3.3k-ohm, instead of 1k-ohm). Which lead to an idle bias of 72-75mA per tube, at 472VDC. This was after a tech R&R the amp.

Also, notice the big dimple on the PT? Sign of possible heat damage.

The photo also shows the heat damage to the heater wires. Coming from the pilot lamp to the 6L6GCs.

Second photo shows the 20-amp Littlefuse which had been replaced some time ago into the amp. Note, the glass is cracked.

Third photo shows heat adamge to wirign under the doghouse.


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Post subject: Re: Toasted SF Dual Showman
Posted: Fri Mar 15, 2013 6:13 pm
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"Sauce for the goose, Mr. Savak."

You love a challenge, Steve.

:mrgreen:

Arjay

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Post subject: Re: Toasted SF Dual Showman
Posted: Fri Mar 15, 2013 7:01 pm
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Aspiring Musician
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Retroverbial wrote:
"Sauce for the goose, Mr. Savak."

You love a challenge, Steve.

:mrgreen:

Arjay


Apparently he does, for which I am extremely grateful.

I am the owner of said Showman. No, I did not put the 20 amp car fuse in the amp. I have owned the amp since 1976.

I think Steve is doing a wonderful job on my amp and no one could ask for a better amp tech. In two days he sent me 13 e-mails, plus 3 phone calls and 11 photos of the progress he is making on my amp.

His attention to detail is amazing and I have no doubt my amp will perform like new when he gets finished.

Thanks you so much Steve.

Thanks to you also Arjay for all your contributions.

Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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Post subject: Re: Toasted SF Dual Showman
Posted: Sat Mar 16, 2013 6:17 am
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Jerry & Arjay,

Thanks, for the compliments

Gonna crash after 3 very crazy days at work. 5 Code Blues in three days. :(

Sure does get in the way of my amp work. Though, the female eye-candy at University/Norris Hospital almost seems worth it. Almost. :lol:

When awake, I'll replace the main power supply diodes with Fairchiid sintered glass UF5408 diodes. And the rectifier dioce with similar UF4007.

Rewire the heaters. And try the amp again. I have gotten a source for the PT and OPT (output tranny).

Here's a photos of the out-of-spec resistors and caps I've already replaced. And six new power train UF5408 diodes and one UF4007 bias supply diode.


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Post subject: Re: Toasted SF Dual Showman
Posted: Sat Mar 16, 2013 10:58 am
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Funny I should be working on my DSR when I spot this post. I was wondering if anyone could tell me what the orange wire wrapped around the red/yellow wire is for on the PT.

Thanks

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Post subject: Re: Toasted SF Dual Showman
Posted: Sat Mar 16, 2013 11:17 am
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Funny you should ask...

I don't believe that the power tranny is original in this Showman. Notice that there are TWO lines from the PT to ground. An orange one and a red-yellow one. This should only be in an amp that has the center-tap (CT) of the heater line grounded. And not one with two 100-ohm balancing resistors on the pilot lamp. Having the heater CT grounded AND the two 100-ohm resistors effectively cuts the heater voltage in half.

The Red-Yellow line is the CT of the main voltage going to the rectifier. It is properly grounded.

The schematic & layout of the AB763 Showman's heater setup versus the AA768 ---you can see the taps off the PT at the lower right corner of the schematic. Notice how the AB763 has an internal ground point on the heater taps. And the AA768 doesn't?

And how this AA768 Showman has 100-ohm resistors from the heater line/pilot lamp to ground and the AB763 doesn't?

The Orange line is an external center tap off the heaters. And should not have been connected with those 100-ohm resistors in place. These resistors shows severe signs of heat damage, esp on the non-Fender solder point to the ground. this is prolly due to excessive current draw to compensate for the low heater voltage --- due to the miss-wiring.

Amp has a lot of heat damage.


http://www.webphix.com/schematic%20heav ... _schem.pdf

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Toasted SF Dual Showman
Posted: Sat Mar 16, 2013 7:12 pm
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I think this is an AA769 you have there because my DSR is a 69' and the schematic shows the two 100 ohm resistors on the heater lines and the 3.3k on the bias supply line. I'm also guessing since my PT looks the same wire wise as your PT that it must be an original tranny unless mine was swapped out with one identical to yours.

Anyway when I ohm out the orange wire I get 10 meg or so to any other PT wire or ground so it's open which makes sense with the 100 ohm resistors in circuit. The whole reason I ask is because this wire broke off the ground connection in my amp and I was wondering what it could be for.

Please slap me now if I'm derailing this thread.

Thanks

Arc

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob


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Post subject: Re: Toasted SF Dual Showman
Posted: Sat Mar 16, 2013 8:55 pm
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No problems with your postings. Good learning session. Thanks!

Problem is the amp I have is running around 470 VDC on the anode plates and not 405 VDC (like the AA769). It's running voltages closer to the AA768.


Check the orange line's continuity to the shell of the PT. In maybe connected to the internal ground of the PT.



http://www.webphix.com/schematic%20heav ... _schem.pdf


http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Toasted SF Dual Showman
Posted: Sat Mar 16, 2013 9:51 pm
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The orange wire is a shield line. Which apparently is not connected to the case of the PT (I tried testing it). Apparently hooked to an internal isolation shield inside the PT, without continuity to the case.

I found this out on another board.


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Post subject: Re: Toasted SF Dual Showman
Posted: Sun Mar 17, 2013 5:45 am
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Thanks for that. I was hoping the orange wire was for SOMETHING! If I didn't have my DSR apart right now I'd do a voltage check for you but if I recall the last time I looked at mine they were all pretty close. The problem I have with my DSR is that I can easily tolerate the volume up at 10 where I can't tolerate my Marshall 50 watt head past 2. The DSR sounds great but I haven't figured out why the output is so low. I figured I get started on it now but first I want to get the chassis straightened out. Somewhere in this amps life it took a spill and the trannys yanked the chassis out from flat.

I hope you can give me some advice as you move along with your repair and I appreciate what you have helped me with already.

Thanks again.

Arc

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob


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Post subject: Re: Toasted SF Dual Showman
Posted: Sun Mar 17, 2013 9:02 pm
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Showman Reverb done. Biggest problems were: 1.) a hinky PT (very low voltages --- below 5VAC end-to-end). 2.) New rectifier diodes in the main PS (Fairhchild UF5408 works very nice for these amps. Redone bias power supply.

3.) Replaced the grid-to-ground 68k-ohm resistors on the 6L6GCs, the 47k-ohm resistors on the PI plates. Most couplling caps --- several of those generic OEM brown drop passed more than a 1-2 VDC. And the components under the doghosue.

4.) Sockets showed sign of heat damage. Brittle, with pieces falling off. UNfortunately, this later era SF had 1-inch socket holes & it's hard to find good NOS 1-inch sockets. So had to use Russian made ceramic ones.

5.) R&R the heater lines. And new 100-ohm resistors on the heater-to-ground pseudo-CT. OEM ones read 225 and 310 ohms.

5.) Oh, I put in 2-amp Slow-Blow fuse (amp rec 2.5 amps).

Plate voltage is 468 VDC at the anode. And the idle bias reads -50.2 VDC at the those 68K-ohm resistors. Idle bias current trhough EH 6L6GC = 40-43mA per tube at 468 VDC.

Equals around 19.5-19.8 watts idle dissipation per tube.

Tone is very clean. Loud (though a four by 10-inch cab). A little "tight." Prolly due to components still breaking-in.

See how this amp behaves, over time. :mrgreen: :D :mrgreen:


http://www.webphix.com/schematic%20heav ... _schem.pdf



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Post subject: Re: Toasted SF Dual Showman
Posted: Sun Mar 17, 2013 9:08 pm
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MIKE ALPHA!

I knew there was hope......with the right tech in command.

Congratulations, Steve. I know Jerry will be tickled to death.

RAWK ON!

8) 8) 8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Toasted SF Dual Showman
Posted: Mon Mar 18, 2013 12:03 am
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Yes, Jerry is Very Tickled :mrgreen: :D :mrgreen: :D :mrgreen:

Steve really knows what he is doing.

No one could ask for a better amp tech. He kept in contact very regularly and took great pains to describe to me, in detail, everything that he was doing and why. I got many many e-mails, photos and phones call throughout the whole process. On my side of this situation, it was a very pleasant experience and nobody could expect or ask for more.

I am super satisfied !!

Plus, he is a hell of a nice guy too !! 8) 8) 8)

Thanks so much Steve.


Jerry

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'62 Jazzmaster/'78 Precision Bass/'88 Fender F250 acoustic/'07 Gibson Less Paul Classic Antique/'12 Squier Strat/'14 Squier VM Jaguar/'15 Classic 60s Telecaster/'68 Showman Reverb/'70 Bandmaster Cabinet JBL D140F speakers/'69 Super Reverb/'12 G-DEC 3-30


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Post subject: Re: Toasted SF Dual Showman
Posted: Mon Mar 18, 2013 4:33 am
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BMW2002Ti wrote:
The orange wire is a shield line. Which apparently is not connected to the case of the PT (I tried testing it). Apparently hooked to an internal isolation shield inside the PT, without continuity to the case.

I found this out on another board.


Image


I think the 230v and 240v primary wires are the other way round.

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Post subject: Re: Toasted SF Dual Showman
Posted: Mon Mar 18, 2013 8:39 am
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Aaron,

Drawing prolly is correct. When you wire up one of these PT's that are made for various countries voltages and Hz --- I've seen jumpering of specific primary taps wrt to the voltage needed.


http://www.hammondmfg.com/pdf/EDB290NX.pdf


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